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B3104U

UL 60950 3rd Edition (formerly UL 1950, 3rd edition)

UL 60950 Terminology

The following definitions assist in understanding UL 60950:

SELV

TNV

TNV3

TNV2

TNV1

Secondary circuit whose voltage values do not exceed a safe value

(voltage less than hazardous levels of 42.4 V peak or 60 V dc)

Telecommunication Network Voltage (a secondary circuit)

SELV but with exposure to surges

SELV but without exposure to surges

SELV with exposure to surges

Creepage distance is the shortest distance between two conductors, measured along the

surface of the insulation. DC voltages shall be included in determining the working voltage

for creepage distances. (The peak value of any superimposed ripple or short disturbances,

such as cadenced ringing signals, shall be ignored.)

Clearance distance is the shortest distance between two conductive parts or between a

conductive part and the outer surface of the enclosure measured through air. DC voltages

and the peak value of any superimposed ripple shall be included in determining the working

voltage for clearance distances.

Creepage and clearance distances are also subject to the pollution degree of the

equipment:

• Pollution degree 1 — components and assemblies that are sealed to prevent ingress of

dust and moisture

• Pollution degree 2 — generally applicable to equipment covered by UL 60950

• Pollution degree 3 — equipment is subject to conductive pollution or to dry non-

conductive pollution, which could become conductive due to expected condensation.

To ensure safe operating conditions of the equipment, UL 60950 focuses on the insulation

rating of the circuit(s) under consideration. Table 4.15 and Table 4.16 indicate the required

creepage and clearance distances depending on material group, pollution degree, working

voltage, and maximum transient voltage in the secondary circuit. For a typical

telecommunication application with a working voltage of 200 V, pollution degree 2, material

group IIIb, the creepage distance is 2 mm. The clearance distance is 2 mm for reinforced

insulation.

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